Cruregens, (nov. gen.)
Generic characters:—Body sub-cylindrical. Head small. First six thoracic segments sub-equal, the seventh small and without appendages. Antennæ sub-equal, neither having a flagellum. First pair of thoracic legs large and sub-chelate, the second and third sub-chelate but smaller, the, three posterior pairs simple. First pair of abdominal appendages forming an operculum enclosing the branchial plates; last pair biramous. Telson squamiform.
In the antennæ, the shape of the body and of the thoracic legs, this genus resembles Paranthura, Spence Bate, and in the shape of the telson and the last pair of abdominal appendages, it is like the closely allied genus Haliophasma, Haswell, but it differs from both in the absence of the last pair of thoracic legs.
Cruregens fontanus, sp. nov. Pl. X. figs. 1 to 12.
Eyes none. Short blunt rostrum between the bases of the upper antennæ. Upper antennæ slightly shorter than the lower, formed of four joints; first joint of lower antennæ long, second short third and fourth about as long as the first, the fourth being followed by a short terminal joint. First pair of thoracic limbs strong and sub-chelate; hand large, broadest at the proximal end, narrowing distally, the palm armed with stout spines, the finger strong and slightly curved; the wrist about twice as long as broad. Last pair of abdominal appendages two-branched, first branch consisting of a single long narrow joint, the other of two joints, the
basal one broader and longer than the terminal one. The ends of both branches supplied with numerous rather long setæ. Telson squamiform, sides arched, ending in a blunt point tipped with a few short setæ. Colour transparent. Length of largest specimen .46 inch.
Hab. Pump at Eyreton.
As this Isopod is exceedingly interesting, I have endeavoured to describe and figure it in some detail. The mouth parts are all small and exceedingly difficult to make out, owing to the various parts being to a considerable extent anchylosed together; and I have to thank Professor Hutton for valuable aid in their dissection. Though I have by his aid succeeded in making out the various parts which form the mouth, I cannot, in the absence of sufficient books of reference, be quite certain of their homologies. All the parts of the mouth project forwards; just below the antennæ there is a strong moderately sharp labrum or upper lip (pl. X., fig. 3). The appendage which, by its position, should correspond to the mandibles does not appear to perform the function of mandibles. The two parts, right and left, appear to be here anchylosed together, and no trace of any palp is to be seen; the distal ends are fringed with very short indistinct setæ, and the opposite end is notched (fig. 4). The first maxilla is simple, spoon-shaped at the end, which is fringed with setæ (fig. 5). The second maxilla is also simple, straight, and towards the end it is supplied with short teeth (fig. 6). Though this appendage, by its position, appears to correspond to the second maxilla, yet it is the only one that looks at all like a masticatory organ. If the right and left halves were rubbed longitudinally together they would, owing to the short teeth at their ends, form a most efficient triturating organ. The most posterior of the mouth organs, the maxillipedes (fig 7), are somewhat concave and operculiform, enclosing the rest of the mouth parts. The basal parts of the two halves on the two sides are anchylosed together, and to the head itself, but the terminal portions are free, and the ends, which are at some distance apart, are tipped with setæ. On each side, near the middle, is a peculiar looking small round piece, which is articulated to the rest of the limb. This may possibly represent a rudimentary exopodite, while the free terminal portion may be the endopodite of the typical Crustacean limb.
The appendages of the abdomen on each side consist of a short basal joint supporting two more or less oval branchial plates (fig. 12). In the appendages of the first abdominal segment the outer branch is much enlarged and forms an operculum over the branchial plates; the inner branch, which lies under the operculum, is narrow (fig. 11). In the view of the abdomen from below (fig. 10), the two halves of the operculum have been slightly separated from one another to show the branchial plates underneath.